An interface device for connecting a cellular radio transceiver to a conventional telephony device is not new. For example, U.S. Pat. No. 4,658,096, naming West et al as inventors, describes an interface arrangement for connecting a conventional telephone set to a cellular transceiver such that the cellular network can provide telephony service to such a telephone set. This patent describes an interface which includes means for automatically determining when the user of such a telephone set has finished dialling, as the concept for connecting a telephone to a cellular transceiver was known in the "radio-patch" art. The above cited West patent is known in the art, and has been cited many times in subsequent patents dealing with further aspects of connecting a cellular transceiver to a telephony device.
There exists a known problem when such a telephony device is connected to a cellular transceiver. Typically an extension cable connects the cellular transceiver interface to the extension device. The part of the extension cable in the vicinity of the cellular transceiver will pick up modulated RF signal transmitted by the transceiver, producing an induced signal in the extension cable.
This induced signal will travel along the extension cable into the extension device where it be demodulated. The resulting demodulated signal will be heard as a background noise on the extension handset. This demodulated signal will also travel back into the cellular transceiver, wherein the signal will be heard as background noise if a handset is connected directly to the transceiver. Note that the RJ-11 port of the cellular transceiver can be fitted with an RF filter to prevent the induced RF from entering the transceiver directly. However the demodulated signal from the extension device will not be blocked by such an RF filter.